Stonnie Dennis is my favorite in terms of what I've actually learned from youtube trainers. I'll preface this that I don't agree with everything he says (though I'd say the same for any trainer on youtube). But I think his general approach to dog training has taught me so much, and has been way more influential and useful than any other trainer. He teaches more about how dogs think, and how to build a relationship with the dog you are training. It's not a recipe book. He focuses on incremental improvements and appropriate goal setting that are much more realistic to put to use in my own life. Not to mention that his videos are just super entertaining to watch. And each one is super long and thorough. You won't find things like "how to teach a dog to stay" but you will find things on how to make yourself more interesting to your dog, how to introduce your dog to different experiences, and how to deal with high distraction environments. The specific things he just teaches along the way.
That said, what's important is finding a trainer whose philosophy works for you and your dogs. Because a trainer is only useful if they bring real improvements to you and your dog.
